1. Field of the Invention
The present invention relates to a Device-to-Device (D2D) network. More particularly, the present invention relates to a method and apparatus for performing synchronization in a D2D network.
2. Description of the Related Art
In recent years, data traffic has been dramatically increasing due to the prevalence of mobile devices such as smart phones. A report of the Korea Communications Commission (KCC) shows that mobile data traffic has increased three times over the past year due to the accelerating prevalence of mobile devices. In the future, the number of users of the mobile devices will further increase and the application services running in the mobile devices will be more prevalent, so the data traffic is expected to increase much more. In particular, if Machine-to-Machine (M2M) communication utilizing machines, such as communication between people and machine, and communication between machines, which is a new mobile market, is prevalent beyond the communication between people, the traffic transmitted to an evolved Node B (eNB) or Base Station (BS) is expected to surge.
Therefore, technology capable of solving these problems is required. Recently, Device-to-Device (D2D) direct communication technology has attracted a lot of attention. This technology, called ‘D2D communication’, has attracted attention in both licensed bands of mobile communication and non-licensed bands of, for example, Wireless Local Area Network (WLAN).
D2D communication is noteworthy in that if converged in mobile communication, the D2D communication can increase the traffic capacity of the eNB and reduce the overload of the eNB. In other words, if User Equipments (UEs) or Mobile Stations (MSs) in the same cell or cells adjacent to each other directly exchange data over a D2D link without passing through an eNB after setting up the D2D link between the UEs, the number of links may be reduced from 2 to 1, contributing to the resource efficiency.
Research on the non-licensed bands aims to prevent the unnecessary waste of wireless resources by recognizing the requests for communication between people, communication between people and machine, and communication between machines, and to appropriately provide a service by determining the traffic that occurs locally. Therefore, the research is focused on a method of efficiently operating a process of broadcasting information about service and content to the surroundings and receiving the service and content by a plurality of devices.
In order to perform a method of controlling communication between devices, a process of setting up synchronization between devices is required. Setting up synchronization between devices using time information received via a synchronous eNB or a Global Positioning System (GPS) receiver module can be considered. According to this method of setting up the synchronization between devices, by a device, synchronization using time information received via a synchronous eNB or a GPS receiver module, the device is required to access the synchronous eNB or the GPS receiver module. However, in a certain communication scheme provided by communication operators, the device may not support the synchronous eNB, so it may not set up synchronization using the time information received via the synchronous eNB. In addition, if the device is located in a GPS shadow area (for example, an area between the skyscrapers, a tunnel area, an interior of a building, and the like), the device may not set up its synchronization since it cannot receive time information from the GPS. As such, the device may not even initiate the D2D communication, if its access to the synchronous eNB or GPS receiver module is not smooth. Besides, some devices may have a high-battery consumption problem, since they consume a lot of power when equipped with a GPS receiver module.
Setting up synchronization between devices without the help of an apparatus that transmits reference time information, such as the GPS and the synchronous eNB, refers to matching the reference times of the devices. Each device has an oscillator, and sets its reference time based on the oscillation frequency of the oscillator. In order to efficiently exchange control signals and data signals with each other in a D2D communication link without collision, all devices need to operate in the same frame structure as they have the same reference time.
However, in the initial phase, the devices may have different reference times since the devices drive their oscillator at different times. For example, on the assumption that a value of an oscillator repeatedly increases from 0 to 100, if a device A started its oscillator at an absolute time of 10 and a device B started its oscillator at an absolute time of 40, then the device A has an oscillator value of 40 and the device B has an oscillator value of 10 at an absolute time of 50.
Therefore, in the existing network, the D2D synchronization method allows the devices to share a difference or offset between the oscillator values to adjust their reference time. In the foregoing example, at the absolute time of 50, the device A informs the device B of its oscillator value of 40, and in response, the device B informs the device A that an offset between the oscillator value ‘40’ of the device A and the oscillator value ‘10’ of the device B is 30. Based on the offset information, the device A corrects its oscillator value, thereby matching the synchronization between the device A and the device B.
However, if the number of devices increases, it is difficult to converge the network within a predetermined time by merely repeatedly performing this point-to-point operation. Therefore, several methods have been devised, such as a method of selecting a representative device and operating based on its reference time, and a method of repeatedly performing a point-to-multipoint operation.
The conventional technology is limited to the method in which devices exchange their time information with each other, since it is difficult to support the scalable network with the method of selecting the representative device. Basically, the devices exchange information about an offset in reference time between the devices using a Media Access Control (MAC) message, to match synchronization. In this case, the device performances may degrade due to different types of latencies which occur in a process of encoding a MAC message during transmission and decoding the MAC message during reception. These latencies include encoding latency, propagation latency, decoding latency, processing latency, channel accessing latency, and the like. In particular, when the resource access is performed on a contention basis, the channel accessing latency caused by the collision may be serious. Therefore, in the conventional technology, as the network grows, the complexity of a synchronization method (for example, a method of selecting and managing a representative device) may be higher and the latency may be longer. In a dynamic D2D network in a wide area, the convergence of the synchronization may take a long time.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.